A standard type of hammer drill has a drive spindle that is rotated about and limitedly axially reciprocal along a spindle axis. A chuck body is rotationally fixed on the spindle, but can move limitedly axially on it. This body is formed with an axially throughgoing passage into the rear end of which the drive spindle is engaged and into the front of which the shank of a drill bit is engaged. In addition the body is normally formed with three guide bores inclined to the axis and having inner front ends opening into the front end of the passage and outer rear ends opening at a radially outwardly open groove on the body. Respective jaws slidable in these guide bores have front ends engageable radially with the shank of the drill bit and rear ends formed with radially outwardly projecting teeth exposed in the groove. The jaws hold the drill bit so it is centered on the spindle axis, and the front end of the drive spindle acts as a hammer on the rear end of the drill bit. The spindle rotates and axially reciprocates slightly, with the rotation being transmitted via the body and jaws to the bit and the reciprocation being effective directly on the bit.
The jaws are normally urged radially outward by springs and are displaced radially inward by an adjustment ring which is provided in the groove and which has threads that mate with the teeth on the jaws. This ring is formed of a pair of semicircular parts secured together by a knurled sleeve. As a rule the radially inner surface of the ring rides directly on the floor of the groove, so that this ring is supported radially in front of and in back of the apertures where the guide bores open into the groove.
Since the bit moves axially somewhat it is impossible to seal tightly around it, for example with a rubber collar, to prevent chips and dust created by the drilling operation from getting into the chuck. When drilling overhead the particles cut from the workpiece being drilled into therefore sift down and into the chuck. Obviously these particles are extremely detrimental to the tool, so that periodically the chuck must be taken apart and cleaned, an operation that can only be entrusted to a good mechanic. If the particles are not cleaned out frequently the chuck will rapidly wear out; even if frequently removed they will substantially shorten the life of the chuck.